1. Field of the Invention
The present invention relates to systems and methods of controlling instruments and other payloads mounted on satellites, and in particular to a method and system for directing a satellite downlink antenna using measured orbital ephemeris data such as satellite position and attitude.
2. Description of the Related Art
Satellites, particularly those in geosynchronous or geostationary orbits are an economical means for transmitting information over vast distances. FIG. 1 is a diagram of a typical communication satellite 102 includes a transmitting instrument 104 (such as the downlink phased array antenna) and a receiving instrument 106 (such as the uplink offset reflector antenna) for two-way communication. Typically, the satellite (or spacecraft""s) 102 attitude is controlled to maintain the pointing for one of the instruments, usually the receiving instrument 106. In such cases, the transmitting instrument 104 will be subjected to a pointing error. This pointing error is caused by a number of factors, including: (1) errors induced by the spacecraft attitude control system (2) spacecraft orbit control errors, and (3) the offset steering of spacecraft attitude for maintaining the uplink antenna pointing. Spacecraft attitude control error is caused by torque disturbances acting on the spacecraft 102 and the finite bandwidth of attitude control system responding to the disturbance torque. Spacecraft orbit control error is caused by several sources, including the inaccuracy of thruster firing during station-keeping maneuvers and extrapolated errors between stationkeeping maneuvers. Offset spacecraft attitude steering is needed to compensate for uplink antenna pointing error caused by thermal distortion within the uplink antenna system itself
If left uncorrected, the aforementioned error sources can result in transmitting instrument 104 pointing errors as large as 0.1 degree or more. An error correction system for a spacecraft antenna pointing to compensate for a transient perturbation in the attitude of the spacecraft is disclosed in Chu, et al, U.S. Pat. No. 5,587,714, which is hereby incorporated by reference herein. The Chu system provides for an improved pointing error, but compensates only one of the many instruments on the satellite (the uplink antenna) for spacecraft attitude control errors. Pointing correction for other instruments, including the downlink antenna, is not provided because of bus motion. Further, the correction supplied by the Chu reference results in potentially excessive angular pointing errors, because no provision is made for satellite orbital perturbations from an ideal orbit.
From the foregoing, it can be seen that there is a need for system and method providing computationally efficient and precise satellite instrument pointing correction. The present invention satisfies that need with a pointing system and method that uses the instrument (or payload) attitude and orbit data to generate error correction for the instrument. The downlink antenna pointing is thus described so that precision instrument pointing can be maintained subject to spacecraft control errors as well as offset steering of spacecraft attitude. The correction of downlink antenna pointing can be accomplished either mechanically in the case of a gimbaled antenna or electrically in the case of a phased way antenna.
The present invention discloses a method and apparatus for directing a first satellite system instrument to a first target. The method comprises the steps of computing the orientation of a first satellite system instrument; computing a position of the satellite using measured orbital data; computing a target line of sight (LOS) vector direction from the satellite to the target using the computed orientation and the computed satellite position, and directing the first instrument to the first target according to the computed target LOS vector.
One embodiment of the apparatus comprises at least one attitude sensor for the first instrument and a communicatively coupled satellite navigation system. The satellite navigation system determines the satellite position using measured satellite orbital data. The satellite navigation system computes the orientation of the first instrument using the attitude measurements, and computes the satellite position using measured satellite orbital data. The satellite navigation system also computes a target LOS vector direction from the satellite to the target using the computed orientation and the computed satellite position, and generates a first instrument orientation command from the computed target LOS vector direction. The orientation command is used by the first instrument to direct the instrument in the proper direction.
Another embodiment of the apparatus comprises a first instrument attitude sensor communicatively coupled a first processor and a second processor. The first processor computes an attitude of the first instrument and an ephemeris for an orbit of the satellite. The first processor also uses these computed values to generate the first instrument attitude error estimate and an orbital error estimate. The second processor computes a first instrument pointing error correction using the first instrument attitude error estimate and the orbital error estimate.